Plastic molded parts are used in many sectors. As hollow bodies, they are used, for instance, in automotive engineering as fuel tanks or as reservoirs for other liquids. Since plastic containers are relatively easy to shape, light in weight and also corrosion resistant, plastic containers are a preferred means for storing liquids. They are expected to be mechanically stable, have a low weight and meet the increasingly strict requirements relating to efficient packaging in automotive construction.
Normally, the plastic containers are made by means of rotational molding using a rotational mold. In a familiar production method, a weighed quantity of plastic material in the form of powder, pellets, micropellets or the like is placed as the starting material into a hollow mold whose inner surface will define the outer surface of the plastic container. The mold is then made to rotate around two axes that are usually arranged perpendicular to each other. Heat is introduced into the rotational melt mold. The rotational speeds of the rotational melt molds are so slow that centrifugal forces have very little effect as compared to the force of gravity. The plastic material begins to melt and to adhere to the inside of the rotational melt mold, thereby imparting the plastic container with its later shape. This very widespread variant of the rotational molding process makes use of thermoplastics such as polyethylene (PE), polypropylene (PP), polyamide 6 (PA6), polyamide 11 or 12 (PA11, PA12), polycarbonate (PC) or the like. The processing temperatures have to be above the melting or softening temperature of the plastic material in question.
Some plastics, especially thermoplastics having very high melting or softening temperatures, for example, PA6, or else thermoset plastics, which are by nature not conducive for thermoplastic processing, are preferably processed by the rotational molding process in a likewise known manner in such a way that, as the starting material, a chemical precursor of the material provided for the molded part, the so-called plastic precursor, is placed into the rotational melt mold as a melt in liquid form, where, under rotation while simultaneously being shaped or formed, the melt reacts chemically, especially polymerizes, to form the final plastic material. This method is advantageously used, for example, for the production of molded parts made of polyamide 6 (PA6), polyamide 12 (PA12) or their copolymers, whereby the corresponding lactams, in other words, for instance, caprolactam and/or laurolactam, are used as plastic precursors that are present in solid form at room temperature under normal conditions, but that are processed by means of the rotational molding process in the form of a melt having a very low viscosity (order of magnitude of 10 mPa·s, that is to say, approximately the same as that of water). This variant of the method allows the production of plastic molded parts while avoiding the high temperatures required for thermoplastic processing, and the process temperature is preferably kept below the melting temperature of the finished plastic.
The rotational molding process also makes use of the polymerization reactions of dicyclopentadiene (DCPD) to form poly-dicyclopentadiene (PDCPD, e.g. TELENE made by Rimtec Corp.) or of cyclic butylene terephthalate (e.g. CBT made by the Cyclics company) to form polybutylene terephthalate (PBT). Moreover, it is a known procedure to use the rotational molding process to manufacture molded parts out of polyurethanes (PU) by reacting diisocyanates and/or polyisocyanates with diols and/or polyols as the plastic precursors.
The just-mentioned material systems have in common the fact that the produced molded part is made of a plastic material that is only formed during the forming process, also called the shaping, in the rotational mold, from a starting material in the form of a plastic precursor that is initially present in more or less liquid form in the rotational mold and that reacts chemically, especially polymerizes, during the shaping process.
Rotational molding with plastics as the starting material as well as with plastic precursors as the starting material is a generally known process and is described, for example, in the following monographs:    [1] Crawford, Roy J., Rotational Moulding of Plastics, Second Edition, Research Studies Press Ltd., Taunton/John Wiley & Sons Inc., New York, 1996,    [2] Nugent, Paul: Rotational Molding: A Practical Guide, 2001, as well as    [3] Crawford, Roy J., Throne, James L.: Rotational Molding Technology, Plastics Design Library, William Andrew Publishing, Norwich, N.Y., 2002.
Furthermore, the rotational molding method makes use of parts that are joined integrally to the container during the molding process. As a rule, so-called insert parts are made of metal. Screwing points needed in the rotated plastic product can be affixed on the inside of the mold, for example, as threaded insert parts. These insert parts, also called inserts, are embedded into the wall of the molded part during the shaping process, thus forming a sturdy connection with it after having cooled off. In contrast, so-called integral parts are generally made of plastic or else of fiber-reinforced plastic composites. In contrast to the insert parts, which are merely embedded into the wall of the molded part but without themselves forming part of the wall, integral parts constitute a part of the wall in the later molded part. In the simplest case, insert parts and integral parts are attached by a screw to the inner wall of the rotational melt mold before the forming process.
In actual practice, not only snap-on systems but also magnetic holders are used that hold the metallic insert parts in position in the rotational melt mold.
A fundamental problem in the production of containers is that the melt mold only defines the outer contour of the molded part, but not its inner shape. Even though a theoretical mean wall thickness can be established for the molded part during the production by suitably coordinating the added quantity of material with the size of the inner surface of the mold, it cannot be guaranteed that the container will exhibit a uniform wall thickness. The wall thickness is always subject to a certain variation. Precisely in the area of inner radii, that is to say, in the areas where the wall of the rotational mold protrudes into the interior of the mold, wall thicknesses are obtained that are, at times, actually considerably less than the mean wall thickness. The smaller this inner radius is, the more pronounced this reduction in the wall thickness will be. In contrast, material accumulates in the area of the outer radii, that is to say, for instance, on the outer edge of a plastic container, as a result of which the wall thickness in such areas is greater than the mean wall thickness. As the outer radius decreases, the magnitude of the increase in the wall thickness rises. Whereas outer radii merely lead to an increase in the wall thickness, the stability of thin-walled spots in the area of inner radii can be considerably impaired, thereby diminishing the strength and durability of the molded part.
Special challenges arise in conjunction with complex shapes such as, for example, integrally shaped lugs or the like. Particularly in constricted spaces, for example, in the area of outer walls that run in parallel at a small distance from each other, bridge formation can occur during the course of the rotational molding process, thus promoting void formation between the walls. The envisaged contour feature is then incompletely formed.
Precisely before the backdrop of increasing requirements relating to packaging in vehicles, however, it is often necessary to ideally utilize a complex and convoluted installation space in the vehicle or in a machine, thus entailing a complex container design. Therefore, it is desirable to be able to systematically influence the material distribution, even in molded parts with complicated shapes. In this context, it is advantageous if the wall thickness can be locally increased at specific places in the finished plastic container. Increasing the weighed-in quantity of the added material is a remedy with very limited benefits, since the additionally employed material essentially only leads to a further increase in the wall thickness in the area of the outer radii, while the wall thicknesses in the area of the thin spots are only negligibly improved. In the final analysis, this measure does nothing but increase the consumption of material and the weight of the part, so that, precisely in the case of containers and tanks, the available useful volume is reduced.
It is a known procedure to influence the wall thickness distribution by suitably selecting the rotational speed, the rotational speed ratio, the temperature course in the mold and by employing other measures. U.S. Pat. No. 3,417,097, for example, describes a method in which caprolactam in liquid form is placed into a rotational mold, the caprolactam adheres to the inner contour of the rotational mold while the mold is being rotated, and then polymerizes to form a molded part. In order to improve the uniformity of the wall thickness, it is proposed to divide the amount of material over at least two metering procedures and to employ a predetermined temperature profile and rotation profile. However, here it is not possible to influence the wall thickness in a systematic and localized manner.
Since in the rotational molding method—like with blow molding and in contrast to injection molding—only the outer surface of the molded part is in contact with the mold, the results that can be achieved by such optimization measures with a given geometry of the molded part are fundamentally limited. The more the geometry of the molded part diverges from being spherically shaped, the wider the distribution of the wall thickness. These effects are particularly pronounced during the processing of monomers that are placed into the rotational mold in the form of a low-viscosity melt and that are fully polymerized under rotation, but in principle, they occur in all rotational molding processes and with all material systems employed.
One possibility for locally influencing the wall thickness for rotational molding with plastic powders or pellets on the basis of a thermoplastic sintering process is disclosed in U.S. Pat. No. 6,852,788 B2. This publication describes a composition that comprises carrier and binder components as well as a plastic powder and that is applied as a molding compound into the areas of the rotational mold in which the wall thickness of the molded part is supposed to be increased, that is to say, for example, in the area of ribs and screw domes. Among other things, polyethylene having a very low density, Vaseline, paraffin and beeswax are proposed for use as the carrier and binder components. As an alternative, thermoplastics that have a low melt-flow index and that have been adapted to the base polymer of the molded part can be used.
This method, however, cannot be used when a plastic precursor in the form of a melt is placed into the rotational melt mold and the polymerization of the plastic precursor is carried out and initiated below the melting temperature of the finished plastic. Since the process temperatures are kept below the melting temperature of the finished polymer, the polymer material in powder form that was added along with the composition would not sinter with itself or with the material newly created by the polymerization. Moreover, the production of the molding compound is relatively laborious.